OK! I cannot give you an exact license since it’s already fan art; just credit me and notify me when the game is out (consider it a kind of CC-BY-NC as mentioned in the itch game metadata, except that it only applies to my work of adaptation).
I don’t know if they will work in 4 colors though, you’ll have to try. If it fails, you can always ask pixel artists for help (I’m not 100% satisfied of my sprites, so if you find one maybe they can make nice sprites specifically for this color palette).
To extract the sprites:
- download the cartridge release (.p8.zip) and extract it
- open data_stage_sonic.p8 in PICO-8
- check the spritesheet and run
export sonic_spritesheet.png
If you don’t have PICO-8 I can upload it directly on this page too.